1. Field of the Invention
The present invention relates to a visible-light semiconductor laser device such as a semiconductor laser device of an AlGaInP system.
2. Description of the Prior Art
As a semiconductor laser device having an oscillation wavelength of 630 nm, in a semiconductor laser device of an AlGaInP system has been conventionally studied and developed actively. Since visibility is high in the wavelength of 630 nm, such a device is used for a laser pointer, a line marker or the like. When the device is used for such a product, the device is powered by battery drive in many cases. Therefore, it is desired to develop a device consuming little power.
On the other hand, a light source for recording and reading to and from an optical disk or the like and a light source of a laser printer or the like, requires for a semiconductor laser device oscillating in a wavelength in a visible-light region. Examples of this type of visible-light laser currently used include a semiconductor laser device comprising an active layer having a quantum well (QW) structure on a GaAs substrate.
The above described visible-light laser, requires the shorter oscillation wavelength. A semiconductor laser of an AlGaInP system oscillating in a band of 630 nm is considered for such a device. The decrease in the wavelength makes it possible to significantly improve the recording density in the optical disk or the like and the semiconductor laser of an AlGaInP system can replace He--Ne gas laser having an oscillation wavelength (.lambda.) of 630 nm.
As described in the foregoing, semiconductor laser device, having the active layer a quantum well (QW) structure has been studied. Particularly in order to improve temperature characteristics and characteristics of oscillation threshold current, it is proposed that strains are introduced into a quantum well layer. A semiconductor laser device of an AlGaInP system with a strained multiple quantum well (S-MQW) structure is described in Proceedings of 53th Conference on Applied Physics (Autumn '92), No. 3, 18a-V-10, pp. 954.
For the purpose of less power consumption, however, the oscillation threshold current of the above described device is closely related to the power consumption and is preferably approximately 40 mA or less, and is more preferably approximately 30 mA or less under practically ordinary conditions, for example, conditions under which a device is mounted on a silicon (Si) heat sink fastened on a good heat radiator (a copper block) of, for example, a stem. In the-above described semiconductor laser device having a short oscillation wavelength, however, only an oscillation threshold current of up to approximately 60 mA is obtained under assurance of stable operation at a temperature of 40.degree. C. In addition, when the semiconductor laser device is used for recording and reading to and from an optical disk or the like, it is desired to make the stem small even if heat dissipation characteristics are sacrificed to some extent because of demand for miniaturization of equipment. A small stem having a diameter of from 9 mm and in the future, approximately 5.6 mm is desired. In order to use such a small stem, it is preferred that power consumption is made as small as possible. Moreover, if the semiconductor laser device is incorporated into a system such as an optical disk recording device, temperature assurance of 60.degree. C. is required.
In the conventional device having no strains, it is known that the oscillation threshold current can be gradually decreased by decreasing the cavity length. In such a case, however, it is impossible to increase the reliability, that is, set the maximum operating temperature (the highest temperature at which oscillation is possible) closely related to the reliability to approximately 50.degree. C. or more and preferably, approximately 60.degree. C. or more. Accordingly, a semiconductor laser device consuming little power cannot be put to practical use.
Furthermore, a quantum well semiconductor laser device of an AlGaInP system having tensile strains has been known, which is described in, for example, Proceedings of 53th Conference on Applied Physics (Autumn '92), No. 3, 18a-V-11, pp. 954. In this case, however, an oscillation threshold current is practically approximately 60 mA. It is impossible to set the oscillation threshold current to approximately 40 mA or less when the maximum operating temperature (the highest temperature at which oscillation is possible) is approximately 50.degree. C. or more under the foregoing conditions.